1. Field of the Invention
The present invention is directed generally to an apparatus and method for using perpendicular electromagnetic: fields to produce a plasma and, more particularly, to an apparatus and method for using perpendicular electromagnetic fields forming a lissajous pattern to produce a plasma in a semiconductor processing chamber.
2. Description of the Background
Plasma enhanced processing is well known in the art of semiconductor fabrication. In plasma enhanced processing, a radio frequency electromagnetic field is used to form a plasma within a processing chamber. A plasma is a gas in which electrons have been stripped from their normal orbits around atomic nuclei, so that the electrons easily pass from one atomic nuclei to another. A plasma is formed when a gas is subjected to an intense electromagnetic field or is heated to extremely high temperatures. The plasma can have different effects, such as ionizing particles or creating chemically active species from chemically inactive species within the processing chamber. Plasma enhanced processing is used with various semiconductor fabrication methods, including etching, sputtering, and chemical vapor deposition. Typically, an intense, radio frequency electromagnetic field is used to generate a torroid-shaped plasma within a processing chamber. It is known to generate the intense electromagnetic fields with either a field coil or with electrodes.
One example of a plasma enhanced processing system using a field coil and sputter deposition is described in a paper by S. M. Rossnagel and J. Hopwood entitled Metal Ion Deposition From Ionizing Magnetron Sputtering Discharge, J. Vac. Sci. Technol. B 12(1) 449, January/February 1994. That paper describes an apparatus using plasma enhanced processing to ionize sputtered particles and then to accelerate those particles towards a semiconductor wafer to reduce shadowing during sputter deposition. Ionization of the sputtered particles is accomplished by generating a dense, inductively-coupled radio frequency (RFI) plasma in the processing chamber between a sputter target and the wafer. The RFI plasma is generated with a coil located within the processing chamber between the target and the wafer. The coil generates a radio frequency electromagnetic field which, in turn, generates the plasma from gasses present in the vacuum chamber. The sputtered particles are ionized as they pass through the plasma on their way to the wafer.
Once ionized, the sputtered particles are accelerated towards the wafer with the aid of a bias voltage applied to the wafer. The bias voltage is opposite in polarity to the charge induced on the sputtered particles to attract the sputtered particles to the wafer. The density and uniformity of the plasma depends on the density and uniformity of the electromagnetic field generated by the coil. The desired result is to create a plasma that is both dense and uniform so that sputtered particles are uniformly ionized when they pass through it.
The coil typically has between two and three turns, and the precise shape and length of the coil affects the phase, shape, and strength of the electromagnetic field generated by the coil, and thereby affects the shape and density of the plasma. For example, as mentioned in the Rossnagel et al. reference at page 450, the symmetry of the coil is extremely important when a system having a rotating coil is used. If the coil in such a system is not precisely symmetrical, the moving magnetic field will weakly couple into the RFI plasma and cause variations of the magnetic rotation rate.
Another example of plasma enhanced processing system using a field coil is disclosed in U.S. Pat. No. 5,280,154, issued to Cuomo et al. That design, like the one disclosed in the Rossnagel et al. paper, requires that the coil be precisely shaped for a uniform electromagnetic field to be generated.
It is also known to construct plasma enhanced processing systems that use electrodes to generate an electromagnetic field. Those systems function similarly to the inductively-coupled system described by Rossnagel et al., except that the plasma is generated by electrodes instead of a coil. U.S. Pat. No. 4,887,005, issued to Rough, discloses a three electrode system using two energized electrodes separated by a ground electrode. U.S. Pat. No. 5,543,688, issued to Morita, discloses a system having several parallel electrodes wherein every other electrode is energized and the remaining electrodes are grounded. A plasma is formed between the parallel electrodes and gases introduced into the processing chamber flow between the electrodes and are ionized in the plasma. U.S. Pat. No. 5,039,388, issued to Mayashita et al., discloses a system whereby an energized electrode is provided at the top of a processing chamber and a wafer support forms a ground electrode.
An article by Noboru Nomura et al., entitled Lissajous Electron Plasma (LEP) Generation for Dry Etching and published beginning on page 4332 of the December 1992 issue of the Japanese Journal of Applied Physics, discloses using electrodes to generate a lissajous electron plasma within a processing chamber. That design orients three electrodes with a triangular symmetry around a processing chamber, with a fourth electrode below the others and in the center of the chamber. The reference discloses in .sctn.4, on page 4336, that:
Theoretically, when the phase shift between electrodes is 0.degree. or 120.degree., a symmetric configuration is realized, resulting in a uniform plasma. On the other hand, when the phase shift is set at other values, the voltage drop between electrodes is different [from] one another resulting in a non-uniform plasma.
Apparently, the device disclosed in the Nomura et al. reference is limited to use with phase shifts between the electrodes of 0.degree. and 120.degree.. That portion of the reference also implies that the voltage drop between the electrodes must be identical. In addition, the device appears to require that the frequencies of the electrodes are equal. See page 4332, column 2, lines 2-3. As will be described in more detail hereinbelow, varying the phase shift, amplitude, and frequency allows a lissajous pattern to be varied. The apparent inability of the device described in the Nomura et al. reference to operate when those parameters are varied seems to prevent it from varying the lissajous pattern. The device described in the Nomura et al. reference appears to limited to generating lissajous patterns having a generally triangular shape (when viewed from above), or perhaps a generally circular shape (when viewed from above). See, for example, the first paragraph in .sctn.3 and FIGS. 3(a) and 3(b), both on page 4334. See also, page 4336, second column, lines 3-7. As a result, the device described in the Nomura et al. reference appears to be limited to generating circular-shaped and triangular-shaped electromagnetic fields and, therefore, it has limited applications.
In summary, many prior art plasma generation systems are inadequate for large semiconductor wafers currently being developed. In particular, as wafers approach 30 centimeters in diameter, many prior art systems fail to provide uniform electromagnetic fields across the entire wafer, resulting in non-uniform plasmas, non-uniform deposition on the wafers, and, ultimately, reduced yield. Other prior art systems designed to be used with large wafers appear to have narrow operating parameters and produce only a limited range of plasma shapes. Those systems, therefore, have limited applications.
Thus, the need exists for an apparatus for generating a uniform electromagnetic field over a large area, and for generating the electromagnetic field, and thereby a plasma, in a variety of shapes.